Kazan is the red king's general and his most faithful servant. He was also the one who beheaded the real Tatara.

Spoiler

Kazan was with Shuri since Shuri was a young boy and could not come to terms with hiding Tatara's mother from his king even though Shuri was willing to forgive him. He fell in love with Chigusa (Tatara's mother) and asked her to be his second in Harakiri (a ritual suicide to regain one's honor as a warrior). sources: midori & soaring_wings